But my own personal nomenclature is that a Slip is a flat section. can be beveled, coved, or rounded or just square along the edge. Original purpose was to go behind glass to provide separation from a mounted print or whatever. If it is wider than the rebate - sticking out in other words - it also gives a pleasing visual effect. A lot of old wood-finish frames have a gilded slip. You can also use it on top of the glass purely for the aesthetic aspect. All the wood mouldings you see with gilt sight-edges are all-in-one adaptations of frame styles that would have had a separate slip in the olde days...
Fillets are like slips but have a raised section on the upper edge. These are used mostly nowadays in mounts. Adds a nice visual feature while also increasing the depth of the mount. The raised edge allows you to get perfect alignment. You couldn't use one as a traditional slip, but these are actually better for adding sight edges, especially if you only want 3mm or so reveal. Also handy for extending the width of a too-narrow rebate.
Liners have a rebate. They can be any moulding that fits inside another and provides a sort of visual no-mans-land between art an frame. They can go under glass and are very useful for this purpose. They can be very narrow or very wide, though technically if the liner is wider than the frame it is a Panel. But that splitting hairs a bit.
One of my first customers used to call slips 'stretchers'.
Attaching blind slips.
If they are made of mountboard I usually stick them to the glass with doublestick tape. If they were wood, you could just tack them to the frame with a little dab of PVA. You would mitre the corners anyway so in theory they should stay put without further fixing, but you never know...
No need to fix them too firmly. You might want to remove them later.
